There are many types of optical recording elements known. In many of the elements, the mode of operation requires a recording layer having a high absorption and that the recorded areas, often referred to as pits, have low optical density or high reflection. The high reflection pits are made by ablating away the high absorption recording material, usually exposing an underlying reflective support.
One of the currently popular forms of optical storage of information is the compact disk or CD. Digital information is stored in the form of high optical density marks or pits on an otherwise reflective background, the exact opposite of the above described optical recording materials. In this format, the optical information is most often in the form of read only memory or ROM. Optical information is not usually recorded in real time but rather is produced by press molding. In a typical process, the optical recording substrate is first press molded with a master containing the digital information to be reproduced. The thus formed information is then overcoated with a reflective layer and then with an optional protective layer. In those areas having the deformations or pits, the optical density is higher than in those areas not having the deformations.
It is desirable to produce optical recording media which, when recorded in real time, produce a record that mimics the conventional CD on read out. Read out is at about 780 nm. In this manner, information can be added to the CD and the CD can be used on a conventional CD player.
One recently disclosed system of this type is the so called "Photo CD". In this system, conventional photographic film is first processed in a conventional manner. Then, the images from the film are digitized and the digital information is recorded in a CD readable form on an optical recording material. Images can then be played back on a CD type player into a conventional television. Since a CD has a capacity for a number of digitized images that is greater than the typical roll of consumer film, it is anticipated that the user will want to add images to a preexisting CD. Thus the need for recordable, CD compatible optical recording material.
One method for forming a recordable element that mimics conventional mold pressed CD elements is to provide a heat deformable support having thereon, in order, a layer of a dye that absorbs recording radiation and a reflective layer. Exposure of the recording layer through the support by the recording beam heats the recording layer to an extent that it is said that the surface of the heat deformable support just adjacent to the recording layer surface is deformed. Elements of this type are described in U.S. Pat. No. 4,940,618, European Patent Application 0,353,393 and Canadian Patent Application 2,005,520.
Commercially useful materials of the type described in these references have stringent requirements. The recording layer must be able to couple with incident laser irradiation to provide features having sufficient reflectivity. The layer must also have good stability towards light, heat and humidity for acceptable shelf life. Since the Photo CD is a consumer product, it must be capable of withstanding extreme environments. Between the time the original images are recorded on the Photo CD and the time subsequent images are recorded, the CD might be placed in strong sunlight.
Most often the active recording layer is an amorphous layer of organic dye or mixture of dyes which owe their optical properties to strong anomalous dispersion (absorption) at wavelength slightly less than that of the desired writing laser, leading to a high real component (N) of the index of refraction and high reflectivity. Absorption of light at the writing laser wavelength (controlled by k, also known as the imaginary component of the index of refraction) must be finite but low, to insure a balance between coupling and reflectivity. High values of N in the recording layer allow the absorption of more light into the layer for a given reflectivity.
Dyes having large values of N at the writing laser wavelength (usually at about 780-800 nm in the near infrared) are not common, and those which have appropriate indices and an efficient mechanism for coupling with the incident laser irradiation to provide significant losses in reflectivity are even rarer.
In the U.S. and the European applications mentioned above, the preferred dyes for the recording layer are indodicarbocyanine dyes. However, this type of dye has less than the desired light stability and will in fact fade to an unusable state after only a few days of intense sunlight exposure.
Thus, there is a continuing need for optical recording materials that have the necessary optical characteristics so that they are CD compatible, can couple with incident laser irradiation to form features with sufficient contrast and yet are light stable. It is to a solution to this problem that the present invention is directed.